print more timestamps in find_lsn_for_timestamp (#12641)

Observability of `find_lsn_for_timestamp` is lacking, as well as how and
when we update gc space and time cutoffs. Log them.

pageserver/src/pgdatadir_mapping.rs

@@ -813,6 +813,7 @@ impl Timeline {
         let gc_cutoff_lsn_guard = self.get_applied_gc_cutoff_lsn();
         let gc_cutoff_planned = {
             let gc_info = self.gc_info.read().unwrap();
+            info!(cutoffs=?gc_info.cutoffs, applied_cutoff=%*gc_cutoff_lsn_guard, "starting find_lsn_for_timestamp");
             gc_info.min_cutoff()
         };
         // Usually the planned cutoff is newer than the cutoff of the last gc run,